Turbine overspeed control system

ABSTRACT

Steam flow to a turbine is reduced by partially closing intercept control valves to reduce turbine speed to a reference speed after sudden increases above this speed caused by sudden decreases in turbine load. Circuitry for developing a command signal for closing the valves in response to a difference between the reference speed and the actual speed of the turbine is provided. The command signal magnitude is varied to cause rapid closing motion of the valves during that portion of valve movement having little effect on steam flow and less rapid closing motion during that portion of valve movement having substantial effect on steam flow, thereby compensating for a nonlinear relationship between valve movement and change in steam flow.

(4 1 Oct. 21,1975

l l TURBINE OVERSPEED CONTROL SYSTEM [75] inventor: David Manuel Priluck, Saugus,

Mass.

[73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Oct. 4, 1974 [2]] Appl. No.: 512,808

5/1974 Vogeli 60/660 Primary ExaminerMartin P Schwadron Assistant Examiner-Allen M. Ostrager Attorney, Agent, or Firm-John F. Ahern; James W. Mitchell ABSTRACT Steam flow to a turbine is reduced by partially closing intercept control valves to reduce turbine speed to a reference speed after sudden increases above this speed caused by sudden decreases in turbine load. Circuitry for developing a command signal for closing the valves in response to a difference between the reference speed and the actual speed of the turbine is provided. The command signal magnitude is varied to cause rapid closing motion of the valves during that portion of valve movement having little effect on steam flow and less rapid closing motion during that portion of valve movement having substantial effect on steam flow, thereby compensating for a nonlinear relationship between valve movement and change in steam flow.

5 Claims, 3 Drawing Figures CONTROL m fg WES? L VALVE l4 MAIN CONTROL i VALVE SPEED SENSOR 33 TNTERCEPT CONTROL VALVE CONDENSER GENERATOR REHEAT secnou l I MAIN smP 'QQE QEE l4 MAIN CONTROL VALVE 39, T SPEED SENSOR a 33 T TNTERCEPT CONTROL CONDENSER GENERATOR VALVE REHEAT SECTION FIG. 2

VALVE CLOS|NG"""" out INCREASING SPEED US. Patent 0121,1975 Sheet20f2 3,913,329

(Vout) TO OVER- SPEED CONTROL VALVES 74 FROM TEST SIGNAL GENERATOR FR MSPEED SENSOR v 432- 0 5) FIG. 3

TURBINE OVERSPEED CONTROL SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to turbine overspeed control systems for limiting turbine speed increases during the occurrence of sudden decreases in a load driven by the turbine, where the turbine must be capable of stable operation at the new load. More particularly, this invention relates to an overspeed control system for a reheat turbine generator which must be capable of stable operation upon separation from a large electric system in which generator normally operates synchronously with other generators in the system.

2. Description of the Prior Art In typical prior art reheat turbine overspeed control systems, separate valves are controlled to regulate steam flow to high pressure stages and reheat stages to maintain turbine speed at a predetermined reference speed. A main control valve regulates steam flow into high pressure stages and an intercept control valve regulates steam flow into reheat stages. On the occurrence of an overspeed condition while the generator is separated from the system, by sudden loss of at least part of the load, the main valve and intercept valve are partially closed to decrease steam flow into their respective stages to effect a reduction of turbine speed to the reference speed.

Control systems of the above-described type operate effectively for small load losses or slowly decreasing loads where both valves partially close and the reduced load is proportionately distributed over all turbine stages. These systems also operate effectively on the occurrence of sudden severe load reductions where the load decreases to an amount which the high pressure stages are capable of driving alone. In this case a ram in the intercept valve is actuated causing rapid complete valve closure and a consequent loss of the major portion of driving power normally provided by the reheat stages. Steam flow to the high pressure stages is reduced by the main control valve to that. level necessary to support driving of the reduced load.

On the occurrence of sudden severe load decreases where the load decreases to an amount exceeding the driving capability of the high-pressure stages alone, however, prior art systems of the above-described type become unstable. Once again the ram is actuated causing complete closure of the intercept valve and loss of the load driving power normally provided by the reheat stages. In this case, however, the high pressure stages become overloaded and turbine speed drops below the reference speed causing the intercept valve to reopen. Steam again flows into the reheat stages and the combined driving power of high pressure and reheat stages is applied to the reduced load causing turbine speed to again increase above the reference speed. The intercept valve then alternates between closing and opening movement causing alternate overloading of the highpressure stages and overdriving of the reduced load as the control system hunts for an intercept valve position at which turbine speed will stablize at the reference speed for the reduced load.

The basic cause of control system instability during severe load decreases is intercept valve nonlinearity. That is, when valve movement is caused to occur at a constant rate steam flow changes at a varying rate. In some cases steam flow decreases only during the first of valve closure. Thus, when it is necessary to rapidly decrease steam flow during severe load decreases, prior art control systems which cause closing movement of the intercept valve at a constant rate cannot respond quickly enough to limit increasing turbine speed and the intercept valve must be subsequently rammed closed. If the high pressure stages become overloaded upon loss of the driving power of the reheat stages, prior art control systems begin to reopen the intercept valve. Valve nonlinearity causes a sudden large increase in steam flow as the valve begins to open resulting in the above-mentioned overdriving of the load and consequent speed variation.

The present invention eliminates the instability experienced with prior art systems during severe load changes by improving intercept valve control and eliminating the need for sudden complete intercept valve closure. Thus the speed instability caused by cycling of the intercept valve is avoided. Intercept valve control is improved by development of a nonlinear command signal and application of this signal to the valve causing rapid motion of the valve during that portion of valve movement having little effect on the steam flow and causing less rapid motion of the valve during that portion of valve movement having a significant effect on steam flow.

SUMMARY OF THE INVENTION A command signal controlling intercept control valves in such a manner as to compensate for intercept valve non-linearities is generated by two amplifiers. The command signal is developed at the output of a first amplifier and increases in magnitude at a rapid rate in response to an increase in turbine speed above the reference speed. This rapid signal increase causes rapid initial intercept valve closing motion during that portion of valve movement having little effect on steam flow. At a predetermined speed above the reference speed a second amplifier interacts with the first to cause a reduction in the rate of increase of command signal. This reduction in command signal rate of change causes less rapid valve closing motion during that portion valve movement having significant effect on steam flow. The net effect of varying valve movement in this manner is a rapid, steady decrease in steam flow resulting in effective control of sudden speed increases by intercept valves. The speed instability caused by intercept valve cycling is thus avoided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a steam turbine generator in which the overspeed control system of this invention is incorporated.

FIG. 2 is a curve representing the command signal utilized to compensate for control nonlinearities.

FIG. 3 is a schematic diagram of a circuit for generating the command signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, steam produced in steam gene rater 11 flows through main stop valve 13 and main control valve 14 to a high pressure turbine stage 15. The direction of steam flow is indicated by the direction of the arrows along steam lines. Steam flows through the high pressure stage 15 to reheat section 17 of steam generator 11 where it is reheated and then to intercept control valves 19 and 2]. From valves 19 and 21 steam is applied to a first reheat stage 23, through the first reheat stage and then through second and third reheat stages 25 and 27. Steam from the second and third stages 25 and 27 is collected in condenser 29 where it is condensed and then returned to steam generator 11. The turbine drives generator 31 to generate electric power.

With the turbine rotating at a constant speed, main stop valve 13 and intercept control valves 19 and 21 are full open. The amount of steam required to rotate the turbine at a particular speed is dependent upon generator loading and is normally controlled by main control valve 14 which is governed by control 37 in response to speed changes detected by speed sensor 39. If the speed increases above a predetermined reference speed control 37 applies a command signal to intercept control valves 19 and 21 causing closing movement of the valves until turbine speed decreases to the reference speed.

In accordance with the invention, a command signal is generated which has the characteristic of increasing rapidly during a first period to effect rapid closing movement of the intercept control valves, and then less rapidly during a second period to effect a less rapid closing movement of the valves. These characteristics are illustrated in FIG. 2 V represents the command signal voltage and V, represents the speed sensor output voltage. Each particular V also corresponds to a particular position of the intercept valves. Below reference speed A a zero volt command signal is generated and intercept control valves 19 and 21 remain full open. If turbine speed increases above the reference speed the command signal voltage rises at a rapid rate B proportional to the rate of increase of turbine speed. Closing movement of the intercept control valves at this same rapid rate compensates for the small effect valve movement has on steam flow during initial closure. At overspeed C the valves have moved to that partially closed position where they begin to have significant effect on steam flow and the rate of change of command signal voltage is decreased to a less rapid rate D. Although the valves are also moving toward their closed positions at this slower rate D, they have significantly more control over steam flow and essentially the same rate of change of steam flow as that occurring during rate of movement B is maintained. The net effect is a rapid steady decrease of steam flow until the turbine speed is under control.

FIG. 3 shows a circuit for generating the command signal function of FIG. 2. The command signal is generated at circuit output 41 by amplifier 42 in response to a turbine speed signal applied to circuit input 43. The speed signal is applied through a resistor 44 to a summing node 45 at a first input of amplifier 42. A reference speed signal is also applied to summing node 45 through resistor 46. This reference speed signal is in the form of an adjustable voltage level provided at potentiometer 47 which has one end connected to a positive voltage supply and the opposite end connected to a zero volt reference or ground. A second input of amplifier 42 is referenced to ground. Resistor 48 and potentiometer 49 provide a feedback path from circuit output 41 to summing node 45. Potentiometer 49 is included in the feedback path for the purpose of adjusting amplifier gain. A resistor 50 is connected in series with the amplifier output to effectively increase amplifier output impedance so that limiting amplifier circuitry. to be described, can override the output of this amplifier if the amplifier output exceeds certain predetermined voltage limits.

These voltage limits are provided by two circuits, one including amplifier 51 and the other including amplifier 56. The circuit of amplifier 51 functions to place a predetermined upper limit on circuit output voltage in accordance with a predetermined reference voltage. The circuit output voltage and the reference voltage are applied to a first input of amplifier 51 through resistors 52 and 53, respectively. The reference voltage is provided at potentiometer 54 which has one end connected to a negative voltage supply and the opposite end connected to ground. A second input of amplifier 51 is referenced to ground. Diode 55 is connected in series with the output of amplifier 51 to effectively disconnect the amplifier output from circuit output 41 at all circuit output voltages below the upper limit. If the circuit output voltage at 41 rises above the predetermined limit, an output voltage developed by amplifier 51 decreases sufficiently to forward bias diode 55 and reduce circuit output voltage to the predetermined limit.

The circuit of amplifier 56 functions to place a predetermined lower limit on circuit output voltage in accor dance with a predetermined reference voltage. Normally the lower limit is the zero volt level. The circuit output voltage and the reference voltage are applied to a first input of amplifier 56 through resistors 57 and 58, respectively. The reference voltage is provided at potentiometer 59 which has one end connected to a positive voltage supply and the opposite end connected to a negative voltage supply. A second input of amplifier 56 is referenced to ground. Diode 60 is connected in series with the output of amplifier 56 to effectively disconnect the amplifier output from circuit output 41 at all positive circuit output voltages. If the circuit output voltage at 41 goes negative, an output voltage developed by amplifier 56 increases sufficiently to forward bias diode 60 and increase circuit output voltage to the zero volt level.

An additional input signal to amplifier 42 is provided by amplifier 61 at speeds above C to cause a reduction in the rate of change of circuit output voltage. This signal, which increases at a constant rate with respect to turbine speed, is applied to summing node 45 through resistors 62 and 63. Amplifier 61 is connected in a summing configuration similar to that of amplifier 42. The turbine speed signal from circuit input 43 is applied through resistor 64 to a summing node at a first input of amplifier 61. A reference speed signal representing turbine speed C is also applied to the summing node through resistor 65. This reference speed signal is in the form of an adjustable voltage level provided at potentiometer 66 which has one end connected to a positive voltage supply and the opposite end connected to ground. A second input of amplifier 61 is refering to ground Resistor 67 and potentiometer 68 provide a feedback path from output resistor 62 to the summing node. Potentiometer 68 is included in the feedback path for the purpose of adjusting amplifier gain. Output resistor 62 is connected in series with the amplifier output to effectively increase amplifier output impedance.

In order to prevent the signal applied to resistor 63 from dropping below zero volts when the output from amplifier 61 goes negative limiting amplifier 69 is pro- .videcl. The signal applied to resistor 63 and a reference voltage level are applied to a first input of amplifier 69 through resistors 70 and 71, respectively. The refer ence voltage is provided at potentiometer 72 which has one end connected to a positive voltage supply and the opposite end connected to a negative voltage supply. A second input of amplifier 69 is referenced to ground. Diode 73 is connected in series with the output of amplifier 59 to effectively disconnect the amplifier output from the amplifier 42 input at resistor 63 when positive voltages are applied to resistor 63. When the voltage at the output of amplifier 61 goes negative an output voltage developed by amplifier 69 increases sufficiently to forward bias diode 73 and increase the voltage applied to resistor 63 to the zero volt level.

Provision is made for periodically checking the intercept control valves for proper operation by application of a negative going test signal to circuit input 74. The test signal is applied through resistor 75 to cause a negative unbalance at summing node 45 resulting in development of a command signal at circuit output 41 and consequent partial closing of the intercept control valves.

in operation, the intercept control valve command signal is generated by amplifier 42 in response to the turbine speed signal applied to circuit input 43. The speed signal, which becomes increasingly more negative as turbine speed increases, is algebraically summed with a positive reference speed signal applied to summing node 45 through resistor 46. If the turbine is operating at the reference speed, the turbine speed signal and reference signal balance and a zero volt output signal is developed by amplifier 42. At speeds below the reference speed A the speed signal becomes less negative and the output of amplifier 42 goes negative. Circuit output 41 also attempts to go negative but limiting amplifier 56 prevents this by forward biasing diode 60.

If turbine speed increases above the reference speed the speed signal becomes more negative than the level at which it balances the reference signal causing the circuit output voltage to go positive to prevent an unbalanced condition at the summing node.- The circuit output voltage rises rapidly at a rate proportional to the rate of increase of turbine speed as illustrated at B of FIG. 2. The ratio of these two rates can be set at a particular value by adjusting the gain of amplifier 42 at potentiometer 49.

If turbine speed increases above a predetermined overspeed value C a positive voltage signal is applied to resistor 63 by amplifier 61. Prior to the occurence of this predetermined overspeed a zero volt signal was maintained at the amplifier 42 input at resistor 63 by limiting amplifier 69. At all speeds below C the amplifier 61 output is negative because of an unbalance between the reference signal applied to resistor 65 and the speed signal applied to resistor 64, but limiting amplifier 69 prevents the input signal to resistor 63 from going negative. At speeds above C amplifier 61 applies a positive signal to resistor 63 which increases at a rate directly proportional to the rate of increase of turbine speed. This applied signal assists amplifier 42 in balancing the negative going speed signal at the resistor 44 input causing a less rapid rate of rise of voltage at circuit output 41. This decreased rate of rise of output voltage is indicated at D in FIG. 2. The ratio between the rates of rise of turbine speed and circuit output voltage during transition D can be set at a particular value by adjusting the gain of amplifier 61 at potentiometer 68. The output voltage continues to increase until level E of FIG. 2 is reached at which time limiting amplifier 51 stops the voltage rise.

Aside from limiting circuit voltage levels, the limiting amplifiers effect sharp transition points at each change of the command signal rate. Of particular importance is a sharp transition at point C where the rate of change of valve motion must be altered to compensate for valve nonlinearities. The shapness of these transition points is dependent on the stability of the references utilized to determine these points. Since the references utilized with the limiting amplifiers can be selected to be as stable as desired, by proper choice of positive and negative voltage supplies, very sharp transition points are obtainable.

Circuit input 74 is utilized for the periodic application ofa test signal to input resistor 75. Usually this signal is applied during operation of the turbine at normal operating speed A. The signal takes the form of a negative going voltage and is algebraically added to the actual speed signal applied to resistor 44 to cause a negative unbalance at summing node 45. This negative unbalance simulates an overspeed condition and should cause the intercept valves to close to the degree required to offset the simulated overspeed.

It will be appreciated from the above description that the present invention overcomes deficiencies of prior art turbine speed control systems related to ineffective control of speed during load changes. The effectiveness of typically used intercept control valves is greatly increased allowing rapid speed control during severe load changes without the instability problems experienced with the prior art systems.

I claim:

1. in an overspeed control system for steam turbines of the type wherein steam flow to the turbine is reduced by a valve to decrease speed to a reference speed after sudden undesired increases above this reference speed, the improvement comprising:

a. a circuit for developing a command signal for controlling the position of the valve in response to a difference between the reference speed and the actual speed of the turbine;

. said circuit including means for developing a command signal which initially increases at a rapid rate causing rapid closing motion of the valve during that portion of valve movement having little effect on steam flow and later increases at a less rapid rate causing less rapid closing motion of the valve during that portion of valve movement having substantial effect on steam flow, whereby said circuit compensates for a nonlinear relationship between valve movement and change in the steam flow.

2. A system as in claim 1 wherein said circuit comprises:

a. a first amplifier functioning at speeds above the reference speed to develop a first electrical output signal varying at a rapid rate with respect to turbine speed;

b. a second amplifier functioning at speeds above a predetermined overspeed to develop a second electrical output signal varying at a less rapid rate than the first output signal with respect to turbine speed;

c. means for applying the second output signal to said first amplifier causing a reduction in the rate of in crease of the signal at the output of said first amplimined lower voltage limit 4. A system as in claim 3 further comprising a fifth amplifier acting to prevent the second amplifer output signal from falling below a predetermined lower voltage limit.

5. A system as in claim 2 including means for provid' ing an input to said first amplifier utilized for periodic application of a test signal causing development of an output signal from said first amplifier resulting in partial closure of the valve. 

1. In an overspeed control system for steam turbines of the type wherein steam flow to the turbine is reduced by a valve to decrease speed to a reference speed after sudden undesired increases above this reference speed, the improvement comprising: a. a circuit for developing a command signal for controlling the position of the valve in response to a difference between the reference speed and the actual speed of the turbine; b. said circuit including means for developing a command signal which initially increases at a rapid rate causing rapid closing motion of the valve during that portion of valve movement having little effect on steam flow and later increases at a less rapid rate causing less rapid closing motion of the valve during that portion of valve movement having substantial effect on steam flow, whereby said circuit compensates for a nonlinear relationship between valve movement and change in the steam flow.
 2. A system as in claim 1 wherein said circuit comprises: a. a first amplifier functioning at speeds above the reference speed to develop a first electrical output signal varying at a rapid rate with respect to turbine speed; b. a second amplifier functioning at speeds above a predetermined overspeed to develop a second electrical output signal varying at a less rapid rate than the first output signal with respect to turbine speed; c. means for applying the second output signal to said first amplifier causing a reduction in the rate of increase of the signal at the output of said first amplifier at speeds above the predetermined overspeed; and d. the output signal from said first amplifier constituting said command signal.
 3. A system as in claim 2 further comprising: a. A third amplifier electrically connected to the output of said first amplifier acting to prevent the command signal from rising above a predetermined upper voltage limit; and b. a fourth amplifier also electrically connected to the output of said first amplifier acting to prevent the command signal from falling below a predetermined lower voltage limit.
 4. A system as in claim 3 further comprising a fifth amplifier acting to prevent the second amplifer output signal from falling below a predetermined lower voltage limit.
 5. A system as in claim 2 including means for providing an input to said first amplifier utilized for periodic application of a test signal causing development of an output signal from said first amplifier resulting in partial closure of the valve. 